The present invention generally relates to a device and method for charging electrochemical cells of a multi-cell battery or the simultaneous charging of multiple batteries. In particular, the invention relates to charging a battery with multiple cells wherein each cell includes a third or charging electrode that is used exclusively for charging. Each cell of the battery is charged simultaneously by coupling a third electrode and a bifunctional electrode in parallel with the battery charger. An induction coil distributes and isolates the charging energy to each cell. The cells of the battery, typically wired in series, require no disassembly for charging. The battery charger is be used with a metal-air battery to prolong the life of the cathode but may also be used to charge common two-electrode multi-cell batteries in parallel without breaking the inter-cell connections. It may also be used to charge a plurality of conventional two-terminal batteries wired in parallel.
Generally, battery packs include several battery cells connected in series. Ideally, each of the battery cells within a battery pack will have similar charging, discharging, and efficiency characteristics. However, this ideal scenario is not normally encountered. Thus, a battery pack ordinarily contains several multiple battery cells with each battery cell having different charging characteristics. This condition may produce many problems related to the overcharging and undercharging of the battery cells. For instance, fully charging one battery cell in a battery pack and continuing to charge it may result in overcharging and damage to the fully charged cell. Likewise, ending a charge cycle when only one battery cell is fully charged may result in undercharging one or more of the other battery cells in the battery pack. Therefore, there is a need for a system to provide an isolated charging cycle that accommodates multiple battery cells having varying charging characteristics.
The present invention relates generally to a method and apparatus for rapidly and safely charging a plurality of battery cells from a single power supply. Given the anticipated proliferation of electric vehicles including electric scooters, it will be necessary to have a reasonably standardized recharging apparatus located at, for instance, the vehicle operator""s residence, place of business, parking garage, recharge station, and the like. Additionally, the same design may be used to simultaneously charge multiple batteries for portable devices such as personal computers, cellular phones, and the like.
Generally, there are two types of battery cells. Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. There are many varieties of secondary cells including the common lead-acid and nickel-cadmium (ni-cad) batteries and the less common metal-air batteries such as the zinc-air battery disclosed in patent application Ser. No. 09/552,870, herein incorporated by reference.
Battery packs comprised of metal-air cells provide a relatively light-weight power supply. Metal-air cells utilize oxygen from ambient air as a reactant in an electrochemical reaction. Metal-air cells include an air permeable electrode as the cathode and a metallic anode surrounded by an aqueous electrolyte and function through the reduction of oxygen from the ambient air which reacts with the metal to generate an electric current. For example, in a zinc-air cell, the anode contains zinc, and during operation, oxygen from the ambient air along with water and electrons present in the cell are converted at the cathode to hydroxyl ions. Conversely, at the anode zinc atoms and hydroxyl ions are converted to zinc oxide and water, which releases the electrons used at the cathode portion of the cell. Thus, the cathode and anode act in concert to generate electrical energy.
Metal-air batteries may be charged mechanically and electrically. Mechanical charging is accomplished by physically replacing the electrolyte, the electrodes, or a combination thereof. (For example, see U.S. Pat. Nos. 5,569,555; 5,418,080; 5,360,680; and 5,554,918). Such a charging method requires special equipment, special skills, an inventory of electrolyte and electrodes, and a plan for storing and disposing of hazardous chemicals. Conversely, electrical recharging avoids these disadvantages. The electrically rechargeable metal-air cell is recharged by applying a charging voltage between the anode and cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through a vent.
While clean and efficient, electrical charging of a conventional multi-cell battery does have some other disadvantages. In particular, most multi-cell batteries are designed such that the cells are connected in xe2x80x9cseriesxe2x80x9d such that the discharge voltage between the battery terminals may be increased. For example, a 12-volt battery is normally comprised of 6 battery cells, each producing 2 volts, wired end-to-end in xe2x80x9cseriesxe2x80x9d to provide 12 volts across the two battery terminals. Such a configuration may be electrically recharged by applying a charging voltage across the two battery terminals to reverse the electrochemical process that occurs on discharge. When connected for charging in this fashion, each battery cell wired in series, necessarily receives the identical current flow regardless of its current state of charge and ability to convert this energy to electrochemical storage.
By forcing the same charging current to each cell, charging the battery cells in series is disadvantageous. During the electrochemical recharge cycle of a battery cell, the cell passes through several stages of charging. It is well known that during charging, a phenomenon known as xe2x80x9cgassingxe2x80x9d occurs, that is to say, the battery electrolyte dissociates into gaseous components which may emanate as bubbles. It is usually desirable to reduce the battery charging current during xe2x80x9cgassingxe2x80x9d so as to avoid damage to the electrode which would otherwise be caused by maintaining the charging current at the higher levels permissible in the xe2x80x9cpre-gassingxe2x80x9d or xe2x80x9cbulk-chargexe2x80x9d phase of charging. Thus, it is desired that the battery charger provide a separate charging circuit to each battery cell such that the charging current may by optimized for each stage of a cell""s charging cycle.
It remains that the two-electrode cells of conventional batteries, connected in xe2x80x9cseries,xe2x80x9d cannot be charged xe2x80x9cconventionallyxe2x80x9d through separate charging circuits as the cells are linked end-to-end. Such batteries may charged in parallel by a conventional battery charger only if the inter-cell connections or links are broken or disconnected. In this manner, each cell is independent and separate charging circuits may be attached to each cell.
In the metal-air battery arena, there are two main types of electrically rechargeable batteries. One type includes those with three electrodes for each cell, namely, a bifunctional anode, a discharge cathode, and a charging-electrode (i.e. a third electrode). The discharge cathode is designed to optimize the discharge cycle of the metal-air cell and may be incapable of recharging the cell. Instead, the charging-electrode is used to recharge the metal-air cell. Another type of metal-air cell includes two electrodes, both electrodes being bifunctional. The bifunctional electrodes function in both the discharge mode and the charge mode of the cell, thus eliminating the need for a third electrode. Bifunctional electrodes, however, suffer from a major drawback; they do not last long because the charging cycle deteriorates the discharge system (i.e., bifunctional electrodes suffer from decreasing performance as the number of discharge/charge cycles increase). In some cases as the voltage creeps up the cell may develop a short circuit as a result of a zinc dendrite forming a metallic bridge to the positive electrode, and will consequently cease to function even though the cathode is in good condition and capable of further service.
Thus, tri-electrode cells are advantageous when compared to two-electrode cells in that they offer more stable performance over a greater number of discharge/recharge cycles. In view of the above and the increased availability of tri-electrode batteries, there is a need in the art for an improved battery charging device and method for charging each of the battery cells independently and simultaneously.
The need for an improved battery charging device with cell balancing is illustrated by the following scenario. For example, a battery with four cells intended to be identical typically are not identical for many reasons. In a metal-air cell, the electrolyte may not wet the entire anode thereby leaving useable material isolated. Through cycling, the zinc can become detached from the current collector and become isolated. The resulting parasitic loss may not be equal and some cells will self discharge differently. During discharge, one or more cells will determine the end of discharge. The remaining cells will have some residual energy but cannot be discharged in series as that would damage the empty cell(s). On the next charge cycle, the cell with the most residual charge will be fully charged prior to the other cells, if the cells are charged in series. Further series charging may damage the fully charged cell and the remaining cells are denied a full charge. As the process continues, the cells unbalance further and each cycle capacity is reduced. FIG. 3A shows a set of nickle cadmium cells that are unbalanced by 10 ampere-hours. In this case, 3 ampere-hours is restored to the unbalance after charging.
This invention relates to a parallel charging system for a multi-cell battery wherein each battery cell may or may not include a third charging electrode. The charging electrode eliminates the need to disconnect cells for parallel charging. The parallel charging system disclosed herein can provide the same function. The invention allows for cell charging via the respective charging electrodes without breaking inter-cell connections. Further, the invention provides for isolation of each charging circuit and battery cell by employing an induction core operatively coupled to each cell. Each cell may be connected to the induction core such that the charging current is provided to each cell independent of the other cells via a separate circuit. The induction core automatically balances the current to each cell based on the cell""s ability to draw current. More current is provided to the cells with a lower charge level and voltage.
In the simplest form, cell balancing is achieved using transformer theory. A load on a secondary winding is reflected to the primary winding by the transformer""s turn ratio. Each secondary winding reflects it""s load and if the transformer is tightly coupled, each secondary winding appears as if it were connected in parallel. Each secondary winding draws power as if it was connected to a single supply. This configuration does not require a regulator but just a diode set and a shunt for current measurement. This simple configuration is useful and cost effective as it requires no extra components and performs the required cell balance.
For example, as shown in FIG. 1, the invention allows for parallel charging of a multi-cell battery wherein the battery cells incorporate a separate or third charging electrode in addition to conventional positive and negative discharge electrodes.
The present invention affords for such charging through a charging electrode without the breaking of inter-cell connections. This is accomplished by providing the charging energy through an induction coil so as to provide electrical isolation of each cell. In addition, the present invention may be employed to charge several two-terminal batteries simultaneously regardless of whether they are multi-cell batteries or single cell batteries.
The present invention may also be employed to parallel charge the cells of a conventional two-electrode cell battery without requiring disconnection of the battery cells. As described herein, the isolation of the secondary windings of the transformer allows the parallel battery charger disclosed to be used where a conventional parallel charger would fail.
The present invention also provides a battery charger for charging through a third electrode (i.e. a charging-electrode) used in metal-air cell batteries. Such third electrodes may be formed from a mixture of an lanthanum nickel compound and at least one metal oxide, and support structure. The present invention provides a charging system and method for metal-air cells that provides stable performance over a large number of charge/discharge cycles. The result is improved metal-air battery performance and improved battery life.
Additionally, when employed in vehicular applications, a braking energy recovery device may be included in the charging system to allow for conversion of kinetic energy back into electrochemical storage in a electrochemical/mechanical system.
In one embodiment, the present invention relates to a battery charging system that isolates the parallel charging of each individual cell using an induction coil.
In another embodiment, the present invention relates to a battery charging system that balances the parallel charging of each individual cell using an induction coil and incorporates a current sensor to determine the end charge of each cell and further may include a top-off timer.
In another embodiment, the present invention relates to a battery charger that allows balanced parallel charging to each cell of a conventional multi-cell battery with two-electrode cells without breaking the intercell connections.
In another embodiment, the present invention relates to a battery charging system that balances the parallel charging of each individual cell using an induction coil and recaptures the braking energy for use in battery charging.
In another embodiment, the present invention relates to the parallel battery charging system for simultaneously charging several conventional two terminal batteries in parallel.
In another embodiment, the present invention relates to a parallel battery charging system wherein current and voltage sensors are applied to each charging circuit and are used to control the charging cycle.
In another embodiment, the present invention relates to a method of charging any battery by parallel charging of each individual cell using an induction coil.